R. Sundelin

High Oradient Superconducting Cavities for Storage Rings

Superconducting (SC) cavities for use in electron storage rings have advanced to the point where several laboratories are making definite plans to use them on a large scale. Laboratory tests of multi-cell SC cavities, including input and higher-order-mode (HOM) couplers, have yielded accelerating fields up to 15.3 MeV/m (CW). Reasons for the improved performance are discussed. Since the 1983 International Accelerator Conference in August, 1983, four SC cavities have been tested in storage rings. The cavities have yielded accelerating gradients up to 6.5 MeV/m, in excess of the frequently-quoted objective of 5 MeV/m. In addition, the average gradient obtained in these four tests was 4 MeV/m, almost twice the average obtained in the four cavities tested in storage rings during 1982 - 1983.

Synchrobetatron Oscillation Driving Mechanism

Any cavity-like structure which supports modes that have both a finite accelerating field at the location of a particle beam passing through the structure and a transverse gradient in that accelerating field is capable of driving synchrobetatron oscillations. Such oscillations are incoherent, are substantially independent of the chromaticity, and do not depend explicitly on the ¿-function. Methods of computing the amplitudes of such oscillations, which can be quite significant under some circumstances, are presented.

Superconducting Cavities for Synchrotron Use

For two months, as of this writing, a 60 cm length of niobium S-band standing wave accelerator has been installed in the Cornell Electron Synchrotron. The Q0 has remained at its initial value of 1.1 × 109, and the accelerating field at breakdown has remained at 4 MeV/m. The cavity has been used to accelerate the synchrotron beam to 4 GeV by itself and to accelerate the beam to 12 GeV in concert with the normal RF system.

Parallel Coupled Cavity Structure

A parallel coupled RF cavity structure which provides favorable solutions to all of the requirements for use in an e+ e-storage ring is described. Properties of this structure have been determined mathematically and through measurements on S-band models. An L-band prototype is being constructed and will be tested at high power.

Simulation Measurement of Bunch Excited Fields and Energy Loss in Vacuum Chamber Components and Cavities

Following the method of Sands and Rees, we have developed a two arm comparison apparatus of high sensitivity and good temporal resolution for the measurement of bunch engendered fields. With a relative timing uncertainty of .5 picoseconds the apparatus has demonstrated a sensitivity of .05¿ for the resistive impedance in the presence of a reactive impedance of 4.2¿ for a gaussian shaped bunch of standard deviation 2.1 centimeters. The apparatus is described and results for a typical component are presented. Both energy loss and bunch generated fields are discussed.

3 GHz Superconducting Accelerator Cavity for Use in an Electron Synchrotron

Experience has shown that superconducting accelerator cavity performance improves as the operating frequency rises. To use this effect to advantage in an electron synchrotron one must develop cavities of aperture large enough to provide the necessary phase space admittance and of such a configuration as to avoid problems with lost particles and synchrotron radiation in the orbit plane. The development of such a cavity, open in the mid-plane, is described and test results given.

Fabrication of Superconducting Niobium Radio Frequency Structures

During the last several years a variety of superconducting radio frequency structures have been designed, fabricated and tested. The diverse structures and fabrication techniques will be described, For much of the work described, no claim is made as to the original conception having taken place in our laboratory. This paper is a description of our experiences in this field.

Development of Superconducting Cavities of Cylindrical Symmetry at Cornell

The fabrication, processing and measurement of five single cell and one 5-cell L-band niobium cavities of elliptical shape are discussed. Q-values of 2 to 5×109 have typically been obtained without high temperature firing. In single cells magnetic surface fields as high as 56 mT corresponding to an accelerating field of 7 MV/m have been measured. In the 5-cell structure a gradient of 4.7 MV/m was achieved.

Superconducting Cavities for a Large e+ e- Colliding Beam Accelerator

An L-band cavity accelerator assembly suitable for large storage ring service is being developed. Design criteria are presented and development status is detailed.

Design Studies for a 1500 MHz Superconducting Accelerator Cavity for use in an e+e− Storage Ring

The cost of a high-energy e+ e− machine can be substantially reduced if superconducting cavities with accelerating gradients of several MV/m can be used1,2). From an RF performance as well as economic point of view, higher operating frequencies are preferred. The upper limit, however, is set by requirements of storage ring operation such as useful aperture, beam bunch length, stored energy, and synchrotron oscillation frequency. Based on these considerations, design studies on a 1500 MHz structure have been carried out with a view towards installing 2m of superconducting cavity in CESR. These studies include measurements on higher-mode properties of the multi-cell structure, tests of devices that provide the coupling necessary for higher-mode power extraction, and damping of instability driving modes, multipactor control techniques on superconducting sections, and tests of 1500 MHz multi-cell superconducting sections.